1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, which is mounted, for example, in a vehicle.
2. Description of the Related Art
In a high-compression ratio spark-ignition internal combustion engine, an abnormal combustion sometimes occurs to lead to combustion noise or a fluctuation in output. The “abnormal combustion” here means a phenomenon in which a fluctuation in in-cylinder pressure, which occurs with the combustion, becomes excessively large. As the types of abnormal combustion, pre-ignition and knocking are generally known. The pre-ignition occurs before the implementation of the spark ignition, whereas the knocking occurs after the implementation of the spark ignition.
There are two types of abnormal combustion phenomenon as the pre-ignition. In one type of the abnormal combustion phenomenon, a temperature of an air-fuel mixture (mixture of air and fuel) present in a cylinder is elevated due to compression to cause self-ignition (hereinafter, this type of abnormal combustion is referred to as “compression pre-ignition”). In the other type of abnormal combustion phenomenon, a tip of a spark plug or a deposit becomes a hot spot to cause ignition (hereinafter, this type of abnormal combustion is referred to as “heat-source pre-ignition”). The knocking is considered as a phenomenon in which an end-gas in a peripheral space of a combustion chamber causes self-ignition during a combustion process after the ignition.
The phenomena described above not only cause noise and vibrations but also lead to damages in the cylinder. Ultimately, there is a fear in that the internal combustion engine does not operate. Therefore, there has conventionally been proposed a method of detecting the pre-ignition and determining whether the pre-ignition is the compression pre-ignition or the heat-source pre-ignition so as to implement avoidance means.
For example, JP 2001-159348 A (hereinafter, referred to as Patent Document 1) describes that a factor of occurrence of the self-ignition occurring while the internal combustion engine operates at low rpm under a high load and that of the self-ignition occurring while the internal combustion engine operates at high rpm under a high load differ from each other and therefore the two types of self-ignition may not be avoided by the same avoidance means. In this context, Patent Document 1 describes the following control apparatus. When an effective compression ratio exceeds an allowable amount which enables the suppression of occurrence of the pre-ignition due to the self-ignition, the control apparatus corrects timing of closing an intake valve, which is set based on the degree of opening of an accelerator and a rotating speed of an internal combustion engine while the internal combustion engine operates at low rpm under a high load, to closing timing which makes the effective compression ratio to be an allowable value.
Moreover, JP 11-50892 A (hereinafter, referred to as Patent Document 2) describes a control apparatus for an internal combustion engine, which determines whether the pre-ignition is initial pre-ignition or runaway pre-ignition based on the number of times of occurrence or a frequency of occurrence of the pre-ignition. The initial pre-ignition has a small number of times of occurrence or a low frequency of occurrence and occurs at late timing, whereas the runaway pre-ignition has a large number of times of occurrence or a high frequency of occurrence and occurs at early timing. When the pre-ignition is determined as the runaway pre-ignition, the control apparatus stops fuel supply.
Patent Document 1 describes the compression pre-ignition and the heat-source pre-ignition, and countermeasures for each. As the countermeasures against the pre-ignition occurring while the internal combustion engine operates at low rpm under a high load (compression pre-ignition), means of reducing the effective compression ratio is given. As the countermeasures against the pre-ignition occurring while the internal combustion engine operates at high rpm under a high load (heat-source pre-ignition), means of retarding ignition timing is given. Therefore, the means for avoiding the pre-ignition is switched depending on the engine rpm. However, there is a fear in that the heat-source pre-ignition occurs even when the internal combustion engine operates at low rpm. The heat-source pre-ignition is generally considered to occur as described above; specifically, heat accumulated in the spark plug or the deposit causes the spark plug or the deposit to be the heat source, leading to the self-ignition. When the internal combustion engine operates at low rpm, the heat is released before the heat is accumulated in the spark plug or the deposit. Therefore, the heat-source pre-ignition is unlikely to occur while the internal combustion engine operates at low rpm.
However, the following case is also conceivable. The heat is accumulated in a portion of a wall surface, which has an elevated temperature due to poor cooling, or in a broken portion of any one of spark plugs of a dual ignition system including two spark plugs in the cylinder to generate a heat source. In such a case, it is considered that the heat-source pre-ignition occurs even when the internal combustion engine operates at low rpm. Thus, it is considered whether the pre-ignition is the heat-source pre-ignition or the compression pre-ignition may not be determined based on the engine rpm.
On the other hand, Patent Document 2 describes the determination of two types of pre-ignition based on the number of times of occurrence or the frequency of occurrence of the pre-ignition. According to the method described in Patent Document 2, for the determination of two types of pre-ignition, it is necessary for a vehicle to continue running for a certain period of time under an operating load which causes the pre-ignition. Here, the case where the determination is made based on the number of times of occurrence is described. For example, it is supposed as follows. When the number of times of occurrence of the pre-ignition is ten or more, the pre-ignition is determined as the heat-source pre-ignition. In the other cases, the pre-ignition is determined as the compression pre-ignition. It is supposed that the currently occurring pre-ignition is the heat-source pre-ignition.
FIG. 18 illustrates a time chart in the case where the vehicle constantly runs under the operating load which causes the heat-source pre-ignition. Profiles illustrated in FIG. 18 indicate the operating load, a pre-ignition determination flag, a down-counter for determining the occurrence of the heat-source pre-ignition, and a heat-source pre-ignition determination flag in this order from the top. It is supposed that the pre-ignition occurs when the operating load has a predetermined value or larger. A predetermined load, under which the pre-ignition is caused, is indicated by a dot line in the portion indicating the operating load in the time chart. When the pre-ignition is detected ten times or more as illustrated in FIG. 18 under the constant operating load, the occurrence of the heat-source pre-ignition may be quickly determined.
Because the pre-ignition occurs with the generation of noise, it is easily conceivable that the driver releases an accelerator pedal so as to reduce the operating load when the pre-ignition occurs. Therefore, the operating load does not normally become constant and does not remain under the same conditions which allow the number of times of occurrence or the frequency of occurrence to be counted.
FIG. 19 illustrates a time chart in the case where the operating load is not constant. Profiles illustrated in FIG. 19 indicate the operating load, the pre-ignition determination flag, the down-counter for determining the occurrence of the heat-source pre-ignition, and the heat-source pre-ignition determination flag in this order from the top. As in the case of FIG. 18, it is supposed that the pre-ignition occurs when the operating load has the predetermined value or larger. The predetermined load, under which the pre-ignition is caused, is indicated by a dot line in the portion indicating the operating load in the time chart. Because the operating load is not constant, the pre-ignition is detected intermittently. Therefore, it is understood that the occurrence of the heat-source pre-ignition may not be determined because the down-counter may not constantly down-count the number to zero. Moreover, as the operating condition under which the above-mentioned phenomenon is likely to occur, there is an acceleration state from a state where the vehicle is stopped. Further, it is considered that the heat-source pre-ignition is caused due to a failure as described above, and hence the quick detection of the heat-source pre-ignition is demanded.
Although the methods of determining whether the pre-ignition is the compression pre-ignition or the heat-source pre-ignition as described above are available, there is a problem of incomplete detection of the heat-source pre-ignition while the internal combustion engine operates at low rpm. In addition, there is another problem in that the operating conditions are required to remain unchanged until the determination of occurrence of the heat-source pre-ignition.